Now for my sentiments, I believe there are three key-dimensions: momentum, cloud-support, and people.

Momentum

Rails has the momentum as evidenced by the number of committers and plugins available. Many of the rockstar developers I know are all now developing in Rails, supporting that community, etc. The underlying Ruby language is incredibly expressive and allows dramatic productivity improvements. (write less code that can do more)

Only a handful of rockstars I know are developing on Grails. Typically, those developing on Grails are in enterprises or markets that are comfortable with Java and they want to continue leveraging that investment. (infrastructure, app servers, production support, etc.) Grails/Groovy does have the advantage that It can continue to leverage the output of the Java community (e.g. Spring). The better projects within the Java community are quickly integrated into Grails. For example, Grails uses Hibernate and Spring Security under the hood. As those projects improve, so will Grails.

Cloud Support

Rails and the cloud go hand in hand. Again, of the rockstar Rails developers I know, nearly all of them are running on virtual hardware owned and managed by others. Some of those let others manage the entire application server / platform (e.g. EngineYard). In contrast, all of the Grails development I know of is running on hardware (virtualized) owned and managed by the company itself in their own data centers (or rented space). As mentioned before, this may actually have been the motivating factor to select Grails. That is not to say that there isn’t cloud support for Grails. And now that VMWare owns SpringSource, this may change rapidly now that enterprises will have a trusted name (in VMWare) to host their Grails applications.

People

Rails has the momentum, more committers and potentially a more active community, but lets not fool ourselves -- there is an army of labor out there to sling Java code. Offshore resources are readily available. The market for Rails is developing, but demand isn’t yet high enough to drive consulting companies to substantially increase their supply. As one of my good friends put it, “I’d love to develop in Ruby, but Java pays the bills.”. It is that sentiment, that has may limit the availability of resources.

However, even with readily available Java resources, that doesn’t guarantee the availability of Groovy resources. Both Grails and Rails require a slightly different mindset. If you take a Java developer and tell them to code in Groovy or Ruby, you’ll end up with Java code using Ruby/Groovy syntax. (not good) But at least the tooling, debugging practices, and libraries will all be familiar.

Brian is CTO at Health Market Science (HMS) where he drives innovation and development of their Big Data platform focused on data management and analysis for the Healthcare space. The platform is powered by Storm and Cassandra and delivers real-time data management and analytics as a service.